Proton coordination chemistry in pyrene-based anodes for ultralong-life aqueous proton batteries

Abstract

Sustainable and safe aqueous proton batteries (APBs) have attracted significant attention owing to their unique “Grotthuss mechanism”. Although organic small molecules with stable and adjustable frameworks are promising electrode materials, their easy dissolution in electrolytes and unsatisfactory intrinsic conductivity hinder their broad application in APB devices. Herein, 2,7-diammonio-4,5,9,10-tetraone (PTO-NH₃⁺) with stable intermolecular hydrogen-bond networks was designed via an in situ electrochemical reduction strategy. The optimized molecule structure endows low charge transport barriers, high chemical reactivity, and prominent charge affinity. The fast kinetics of proton coordination/de-coordination behavior in PTO-NH₃⁺ electrodes is corroborated by ex situ characterization techniques and theoretical calculations. As a result, the robust four-step 4e⁻ H⁺ coordination with PTO-NH₃⁺ electrode achieves an excellent rate performance (214.3 mA h g⁻¹ at 0.05 A g⁻¹ and112.9 mA h g⁻¹ at 40 A g⁻¹), along with a long lifespan (10 000 cycles). These findings shed light on further avenues towards advanced proton batteries.

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1. Introduction

With the contradiction between the unparalleled lack of fossil fuels and the increasing global energy demand intensifying, electrochemical energy storage devices, which provide a feasible approach to realizing efficient utilization of renewable energy sources such as solar and wind, have drawn widespread attention.^1–4 Impressively, aqueous batteries stand out for abundant resources, intrinsic safety, and environmental friendliness due to their innoxious and apyrous electrolyte.^5–9 Earth-abundant metal ions (such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, and Al³⁺)^10–28 are selected as charge carriers for aqueous batteries. Nevertheless, owing to their larger ionic radius and charge number than Li⁺,²⁹ it is challenging for these aqueous batteries to achieve competitive performance with lithium-ion batteries.

In view of the smallest ionic radius and eminent abundance in nature, proton (H⁺) with ultrafast diffusion kinetics and negligible cost³⁰ is a considerably attractive substitute for metal cations in the construction of aqueous rechargeable batteries. Since the prototype of a proton battery was first reported by Sjödin's group in 2017,³¹ an increased number of proton storage materials have emerged, including metal oxides (e.g. MoO₃, WO₃, TiO₂ and V₂O₅),^32–35 Prussian blue analogues (PBAs) (e.g. Cu[Fe(CN)₆]_0.63·3.4H₂O, Ni[Fe(CN)₆]_2/3·4H₂O),^36,37 organic solids (e.g. 3,4,9,10-perylenetetracarboxylic dianhydride [PTCDA],³⁸ pyrene-4,5,9,10-tetraone [PTO],³⁹ diquinoxalino [2,3-a:2′,3′-c] phenazine [HATN],⁴⁰ and poly(3,4-ethylenedioxythiophene) [PEDOT]),⁴¹ and MXenes.⁴² Inorganic electrodes often suffer from poor cycle stability, low specific capacity and low redox potential. In this regard, organic electrode materials demonstrate significant advantages, such as structural versatility and functional tunability. However, organic-based proton batteries often suffer from fast capacity attenuation during repeated charge–discharge cycling because of the solubility of the active materials. Furthermore, currently reported organic electrodes for proton storage exhibit slow intermolecular ion diffusion kinetics, resulting in poor rate performance and low redox-active site utilization. Recently, some reports have suggested that intermolecular H-bonds can improve the solvation energy of organic materials, thus inhibiting their solubility.^43–46 In addition, the weak intermolecular hydrogen bond interaction can form a horizontal two-dimensional extended superposition network, thereby improving cycle tolerance and ion transport performance of organic molecules.^47–50 However, an in-depth investigation into the mechanism of hydrogen bond formation between organic electrodes during proton storage is still lacking.

Herein, a planar and highly-delocalized organic electrode material (PTO-NH₃⁺) with stable intermolecular hydrogen-bond networks was synthesized by nitration and electrochemical reduction for aqueous proton storage. Benefiting from its overall molecular rigidity and facilitated aromaticity, the PTO-NH₃⁺ exhibits rapid charge transport and an optimized energy bandgap, resulting in enhanced intrinsic conductivity, high redox activity, and superior electron affinity for proton storage. When served as an electrode material, it demonstrates fast coordination kinetics with an exceptional capacity as high as 214.3 mA h g⁻¹ at 0.05 A g⁻¹, splendid rate performance (40 A g⁻¹), and long-term durability (≥10 000 cycles) in 9.5 m H₃PO₄ electrolyte. Multiple ex situ characterization techniques further revealed that the highly reversible redox centers of PTO-NH₃⁺ upon proton uptake/removal are C=O groups, while the corresponding protonation pathway and structural evolution are validated by theoretical calculations. These breakthroughs pave the way for the design of outstanding organic electrode materials for APBs.

2. Results and discussion

The DNPT sample was straightforwardly synthesized via a one-step nitration reaction of pyrene-4,5,9,10-tetraone (PTO) (Fig. 1a). Fourier transform infrared (FT-IR) spectra of DNPT and PTO were collected, as illustrated in Fig. 1b. Examination of Fig. 1b shows that the peak at 1680 cm⁻¹ is assigned to the C=O bond. The asymmetric and symmetric stretching vibration peaks at 1531 and 1348 cm⁻¹ can be attributed to the N–O bonds of the nitro group (–NO₂). The Raman spectra of DNPT and PTO were also collected. As depicted in Fig. 1c, the –NO₂ and C=O peaks appear at 1349 and 1696 cm⁻¹, respectively.⁵¹ The chemical structure of DNPT was verified by ¹H and ¹³C nuclear magnetic resonance (NMR) (Fig. 1d and e). As shown in Fig. 1d, due to the high symmetry of the molecular structure, the peak at 8.91 ppm corresponds to the hydrogen atom (1) on the pyrene unit. Fig. 1e shows the ¹³C NMR diagram of DNPT, where the five peaks at 149.20, 126.48, 134.56, 136.35, and 174.73 ppm are associated with carbon atoms at 4, 1, 2, 3, and 5 sites, respectively. The morphology of the DNPT sample was observed by scanning electron microscopy (SEM) at low and high magnification (Fig. S1†), revealing its stacked flake structure with a side length of 7–10 μm, and the transmission electron microscopy (TEM) image (Fig. S2†) is consistent with this finding. X-ray diffraction (XRD) patterns were collected in Fig. S3,† likewise corroborating the as-prepared sample is DNPT, while the sharp diffraction peaks demonstrate its fine crystallinity, which can be ascribed to the π–π stacking between pyrene units.⁵² The thermogravimetric (TG) curve (Fig. S4†) was further employed to certify its thermodynamical stability.

Fig. 1 Structural characterization of intermediates. (a) Synthetic process of DNPT. (b) FTIR spectra of PTO and DNPT. (c) Raman spectra of PTO and DNPT. (d) ¹H NMR spectrum and (e) ¹³C NMR spectrum of DNPT (DMSO-d₆).

The electrochemical properties of the DNPT electrode were estimated in a typical three-electrode system utilizing activated carbon as a counter electrode. 9.5 m H₃PO₄ was selected as an electrolyte owing to its prominent conductivity for protons, wherein protons have incomplete solvation shells that enable protons to directly interact with the electrode surface.⁵³ Note that all potentials are relative to the Ag/AgCl reference electrode. The galvanostatic charge/discharge measurements in Fig. 2a show that DNPT can deliver a high initial discharge capacity of 1231.9 mA h g⁻¹ and a charge capacity of 429.3 mA h g⁻¹ at a current density of 0.05 A g⁻¹, giving a coulombic efficiency (CE) of 34.8%. Such an initial irreversible capacity loss can be attributed to the conversion from the –NO₂ group in DNPT to the –NH₂ group.⁵⁴ To gain insights into this reduction mechanism, the DNPT electrodes before/after the electrochemical reaction were further investigated by X-ray photoelectron spectrometry (XPS) to confirm the formation of PTO-NH₂. The high-resolution N 1s XPS spectra are presented in Fig. 2b, indicating that the pristine DNPT electrode presented a characteristic peak of –NO₂ at 406.1 eV, with a satellite peak at 400.5 eV. After discharge, a signal at 399.9 eV is attributed to the amino group (–NH₂) emerges, while the –NO₂ peak completely disappears. During the subsequent cycle, no trace of –NO₂ is detected again, indicating an irreversible reduction from –NO₂ to –NH₂. This point was further confirmed by the FTIR spectra (Fig. 2c) and Raman spectra (Fig. S5†) at different charge/discharge states. As shown in Fig. 2c, two characteristic –NO₂ stretching bands at 1528 and 1346 cm⁻¹ can be observed in the pristine DNPT electrode. After fully discharging to 0 V, the –NO₂ peaks almost completely disappear. Meanwhile, new peaks appear at 3500–3300 cm⁻¹. Among them, the peak at 3430 cm⁻¹ belongs to the symmetric stretching vibration of free –NH₂ (ν_N–H). The peaks at 3360 and 3235 cm⁻¹ correspond to the associated –NH₂ (ν_N–H⋯O and ν_N–H⋯N). The peak at 1586 cm⁻¹ was assigned to the bending vibration of –NH₂ (δ_N–H). The presence of these peaks indicates the formation of –NH₂ groups during discharge. At the same time, the C=O bonds form a hydrogen-bond coordination structure, which transforms into C=O⋯H. When the DNPT electrode was fully charged to 1.0 V, the characteristic peaks of –NH₂ remain, and no –NO₂ signal is detected. Moreover, the difference can be also identified in the crystal structure of the DNPT electrode before/after discharge, which is reflected by the XRD pattern (Fig. 2d). After full discharge, the diffraction peaks of the DNPT electrode completely disappeared, and a new diffraction peak at 27.3° appeared, revealing the formation of a new crystal structure. After a full charge, an obvious peak at 28.6° can be observed, which corresponds to the π–π stacking structure of PTO-NH₂. The morphological evolution of the DNPT electrode during the initial charging/discharging process was also reflected in the SEM images (Fig. S6†). Based on the above analysis results, the DNPT was converted to PTO-NH₂ through the in situ electrochemical reduction method. In acid electrolytes, PTO-NH₂ mainly exists in the form of PTO-NH₃⁺ due to hydrogen bond interactions, as illustrated in Fig. 2e. Furthermore, the structure of the intermolecular hydrogen-bond networks based on the –NH₂ groups in the PTO-NH₂ is shown in Fig. S7.†

Fig. 2 Electrochemical reduction mechanism of the DNPT electrode. (a) Voltage–capacity profile of the DNPT with marked points for the XPS tests and (b) corresponding ex situ N 1s XPS spectra. (c) FTIR spectra of the DNPT electrodes at different charge/discharge states. (d) XRD patterns of the DNPT electrodes at different charge/discharge states. (e) Synthetic process of PTO-NH₃⁺.

The proton-storage metrics of the PTO-NH₃⁺ electrode were evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. The CV curves of the PTO-NH₃⁺ electrode at various scan rates from 0.2 to 1 mV s⁻¹ are collected in Fig. 3a, which displays four pairs of symmetric redox peaks at 0.10/0.12 V, 0.24/0.26 V, 0.38/0.40 V and 0.62/0.64 V, indicating a four-step protonation/deprotonation electrochemistry. The corresponding peak shapes were virtually identical as the scan rates increased, revealing that the PTO-NH₃⁺ electrode exhibits extraordinary electrochemical reversibility. Further investigation of proton storage kinetics is obtained according to the power-law relationship between the peak current (i) and scan rate (v): i = av^b. As illustrated in Fig. 3b, the calculated b-values for the R1/O1, R2/O2, R3/O3, and R4/O4 redox peaks are 0.87/0.96, 0.90/1.02, 0.96/0.97, and 0.99/0.93, respectively, suggesting that the H⁺ uptake/removal process is dominated by capacitive behavior. The hydrogen-bond network between molecules plays a critical role in the energy storage process of organic batteries. The specific mechanisms are as follows: (1) organic small molecules form a network through hydrogen bonds, providing channels for proton conduction and reducing protonation activation energy. The dynamic nature of hydrogen bonds allows protons to transfer rapidly through the Grotthuss mechanism, thereby accelerating the electrochemical reactions on the electrode surface; (2) hydrogen-bond network enables proton conduction to occur primarily over short distances, reducing the need for long-range diffusion and creating localized regions of high proton concentration on the electrode surface, further minimizing the impact of diffusion limitations. Additionally, the corresponding contribution ratios at each scan rate were quantified using the Trasatti equation: i = k₁v + k₂v^1/2. As exhibited in Fig. 3c and Fig. S8,† the capacitive contribution increased gradually as the scan rate increased from 0.2 to 1 mV s⁻¹. When the scan rate reached 1 mV s⁻¹, a capacitive contribution as high as 97.2% was achieved, which was conducive to achieving excellent rate performance. In the same way, we depict the CV curves at different scan rates from 1 to 15 mV s⁻¹ as well as relative b-values and contribution ratios (Fig. S9 and S10†), which reflect conclusions in accordance with those mentioned above. The enhanced capacitive characteristics enabled the PTO-NH₃⁺ electrode to exhibit rapid reaction kinetics.

Fig. 3 Kinetics properties and electrochemical performance of the PTO-NH₃⁺ electrode. (a) CV curves collected at various scan rates from 0.2 to 1 mV s⁻¹. (b) Calculated b values. (c) Contribution ratio of the capacitive and diffusion-controlled charge versus scan rate. (d) Discharge/charge GITT curves for the PTO-NH₃⁺ electrode at 0.1 A g⁻¹. (e) H⁺ diffusion coefficient calculated from GITT. (f) Ex situ EIS plots at different states of charge. (g) GCD profiles at different current densities. (h) Rate capability of the PTO-NH₃⁺ electrode. (i) Lifespan comparison of PTO-NH₃⁺ with recently reported organic materials. (j) Cycle performance under a current density of 10 A g⁻¹ for 10 000 cycles.

To gain further insights into the intrinsic kinetics properties with respect to PTO-NH₃⁺, the proton diffusion coefficient (D_H⁺) and charge transfer resistance (R_ct) were estimated by galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) measurements, respectively. The GITT curve of the PTO-NH₃⁺ electrode is shown in Fig. 3d, and the D_H⁺ results calculated according to the GITT curve are shown in Fig. 3e. As shown in Fig. 3e, the value of D_H⁺ ranges from 10⁻¹⁰ to 10⁻⁶ cm² s⁻¹ over the whole discharge/charge process, confirming the superlative high-rate performance of PTO-NH₃⁺ electrode. Interestingly, during the discharge process from 0.7 to 0.1 V, D_H⁺ essentially maintains around 10⁻⁷ cm² s⁻¹, while subsequent deeper discharge to 0 V gives rise to the rapid decline of D_H⁺. The same occurs during charging, where D_H⁺ rapidly decreases as the potential reaches 0.8 V. To an extent, this phenomenon presumably stems from the critical concentration polarization at the end of the discharge/charge process. Targeting the discharge process, protons can readily coordinate with PTO-NH₃⁺ in the beginning, in stark contrast to the almost filled reaction sites of PTO-NH₃⁺ and thus generate strong electrostatic repulsion at the end of the discharge process, both of which render the diffusion of protons within the electrode more difficult.^55,56 Besides, Fig. 3f compares the ex situ EIS plots at different states of charge, in which the diameter of the semicircle at the high-frequency region denotes R_ct at the electrode/electrolyte interface. The exceedingly minor R_ct values ranging from 2.58 to 2.79 Ω during the discharge/charge process indicate that the PTO-NH₃⁺ electrode possesses low inherent resistance, which is attributed to the extensive π-electron delocalization via constructing hydrogen bond networks.⁵²

The galvanostatic charge/discharge tests were then conducted at various current densities ranging from 0.05 A g⁻¹ to 40 A g⁻¹. As shown in Fig. 3g, the PTO-NH₃⁺ electrode delivers considerable specific capacities of 214.3, 164.8, 152.2, 144.5, 140.8, 137.4, 134.2, 130.3, and 125.4 mA h g⁻¹ at current densities of 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20 A g⁻¹, respectively. Significantly, even when the current density increased 800-fold to 40 A g⁻¹ (i.e., the battery was fully charged in only 10 s), the battery still maintained a decent capacity of 112.9 mA h g⁻¹ (approximately 52.7% of the initial value), indicating its potential use in specialized fast-charging applications. In addition, as shown in Fig. 3h, at a low current density of 0.05 A g⁻¹, the rapid decay of capacity in the initial 5 cycles, may be caused by the side reaction of remanent –NO₂. However, when the current density was increased from 0.1 A g⁻¹ to 40 A g⁻¹, only 31% capacity attenuation occurred, indicating the ultrafast kinetics and superior rate performance of the PTO-NH₃⁺ electrode. By contrast, this rate performance significantly overwhelms most organic materials reported for proton storage (Fig. 3i and Table S1†).^41,57–62 In addition to excellent rate capability, the PTO-NH₃⁺ electrode also manifests promising cycling stability, as evidenced by its capacity retention rate of 85% after 450 cycles at 0.1 A g⁻¹ (Fig. S12†), and still maintains a capacity retention rate of 78% after 5000cycles at 5 A g⁻¹, (Fig. S13†). Impressively, even at an exceedingly high current density of 10 A g⁻¹, the electrode retains 74.1% capacity over 10 000 cycles with a CE of ≈100%, suggesting outstanding structural stability and promising prospects for practical applications of PTO-NH₃⁺ electrodes.

The underlying causes of capacity fade were also thoroughly investigated. As shown in Fig. S14,† the morphology of the electrode changed dramatically, with several floral clusters appearing after cycling, indicating the destabilization of the electrode surface environment. Furthermore, the disappearance of the unique diffraction peak at 28.6°, which corresponds to the π–π stacking structure of PTO-NH₂ in Fig. S15,† shows that the structural stability of the hydrogen-bond network is disrupted, fundamentally contributing to the decay of capacity.

Based on the high capacity and remarkable cycling performance at various current densities, it is evident that PTO-NH₃⁺ exhibited exceptional electrochemical properties as an electrode material for H⁺ storage. Thus, the intermolecular hydrogen bond network makes the PTO-NH₃⁺ electrode have higher structural stability and faster interfacial charge transfer rate, resulting in the PTO-NH₃⁺ electrode having excellent cycle tolerance and rate performance. Meanwhile, Table S1† shows that the cycling stability of PTO-NH₃⁺ is superior to that of most of the reported materials, indicating that hydrogen-bond networks offer significant advantages in suppressing electrode dissolution. Compared with COFs and polymer electrodes, hydrogen-bond networks provide a better balance between dissolution inhibition and electrochemical performance, particularly in high-rate, low-temperature, and flexible applications. These properties make hydrogen-bond network-based organic electrode materials highly promising for next-generation proton batteries.

To clarify the storage mechanism of H⁺, ex situ FTIR and XPS tests were applied to track the structural evolution of PTO-NH₃⁺ electrode at selected discharge and charge states, as marked in Fig. 4a. In the FTIR spectra (Fig. 4b and Fig. S16†), the characteristic signal of C=O bonds in PTO-NH₃⁺ at 1672 cm⁻¹ gradually weakens upon the deepening of discharge. Meanwhile, new absorption peaks corresponding to C–O bonds and O–H stretch appear at 1415 cm⁻¹ and around 3329 cm⁻¹, respectively.^43,56 Moreover, the intensity of these peaks (i.e., C–O and O–H absorption peaks) gradually increases during discharge. This phenomenon indicates that the C=O bonds of PTO-NH₃⁺ are the active sites and are involved in the storage of H⁺ ions. The reversible evolution of the O–H absorption peak also reflects the interaction of the hydrogen-bond networks with H⁺. Besides, during the subsequent recharging process, the intensity evolutions of the C=O, C–O and O–H peaks exhibit the opposite trend to that of the discharge, suggesting that C–O bonds are reversibly transformed into C=O bonds. Furthermore, ex situ XPS spectra of O 1s and C 1s (Fig. 4c and d) were obtained to confirm the coordination chemistry between the H⁺ ions and C=O groups. The O 1s spectra show three peaks at 531.9, 533.1, and 534.1 eV at state I, attributed to C=O, C–O, and O–H species, respectively. The existence of O–H species may be related to the hydrogen-bond networks between PTO-NH₃⁺ molecules. During the discharge procedure (from state I to state VII), the intensity contribution of C=O gradually declines, whereas that of C–O increases. The same trend occurs in the C 1s spectra, where a curve-fitted peak (288.1 eV) corresponding to the C=O bond follows a decreasing path upon proton uptake. Also of note, both O 1s and C 1s spectra can revert to the initial states after recharging (from state VII to state XIII), which is consistent with the ex situ FTIR results. To summarize, carbonyl moieties (C=O ↔ C–O/O–H) of the PTO-NH₃⁺ electrode are uncovered as redox-active centers that prompt the reversible electrochemical reaction upon H⁺ (de)coordination.

Fig. 4 Proton storage mechanism of PTO-NH₃⁺ electrode. (a) GCD profile under a current density of 1 A g⁻¹ with marked points for ex situ tests. (b) Overview of ex situ FTIR spectra. Ex situ XPS spectra of O 1s and C 1s during (c) discharging process and (d) charging process.

To further unravel the effect of the reduction from –NO₂ to –NH₂ on the PTO unit's electrochemistry and the proton uptake pathway in the PTO-NH₃⁺ electrode, we conducted density functional theory (DFT) calculations with frontier molecular orbital theory and minimum energy principle.^63–65 As shown in Fig. 5a, replacing –NO₂ with –NH₂ increases the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) from −8.089 and −4.771 eV to −6.036 and −3.545 eV, respectively. The corresponding energy gap (ΔE) of PTO-NH₂ is reduced to 2.490 eV compared with 3.318 eV in DNPT. The narrower energy gap of PTO-NH₂ is strongly correlated with the higher intrinsic electronic conductivity among organic materials (Table S2†),⁶⁶ which is beneficial to electron transfer when applied as electrode materials. The Gibbs free energy changes (ΔG) of PTO-NH₃⁺ molecules incorporating various numbers of hydrogen atoms and their corresponding electrostatic potential distributions (ESPs) are given in Fig. 5b, from which we can see the electrostatic potential around functional C=O was negative (nucleophilic center) while positive (electrophilic center) around –NH₃⁺, indicating that H⁺ ions favor the active site of C=O. During the protonation process, the protons successively coordinate with C=O bonds in a four-step reaction procedure to form PTO-NH₃⁺-H (−0.6433 eV), PTO-NH₃⁺-2H (−0.7524 eV), PTO-NH₃⁺-3H (−1.9777 eV), and PTO-NH₃⁺-4H (−2.6276 eV), which conforms to the four pairs of redox peaks observed in CV curves. When the PTO-NH₃⁺-4H molecular state is achieved, the continuous blue region on the surface of C=O also almost becomes the red region, indicating that the discharge process is complete. Furthermore, the Gibbs free energy changes for incorporating hydrogen atoms are negative, demonstrating that the reaction should occur spontaneously. To gain more insight into the nature of the bonds formed between C=O and hydrogen atoms, charge-density difference distributions were also estimated. As illustrated in Fig. 5c, the electron density change occurred mainly around the active site of C=O, which agrees with the conclusions drawn from the ESP distribution. According to the Grotthuss mechanism, the transfer of free protons occurs directly in the adjacent hydrogen-bond networks of PTO-NH₃⁺ molecules.

Fig. 5 DFT calculations. (a) HOMO–LUMO plots of DNPT and PTO-NH₂, together shown are their corresponding values and gaps in the unit of eV. (b) Charge density difference for PTO-NH₃⁺, which possesses (i–iv) 1, 2, 3, and 4 hydrogen atoms, respectively. (c) Gibbs free energy change (ΔG) of PTO-NH₃⁺ molecules reacted with different hydrogen atoms, together shown are their corresponding ESP images with a color scale ranging from −50 (blue) to 50 (red) a.u.

3. Conclusions

In summary, a planar organic electrode PTO-NH₃⁺, which forms intermolecular hydrogen bonds through electron delocalization polarization between C=O and NH₂ has been demonstrated for splendid proton storage. Furthermore, the electrochemical mechanism of the PTO-NH₃⁺ electrode was verified to be H⁺ coordination/de-coordination chemistry using ex situ characterization techniques and DFT calculations. The C=O groups with negative ESP are the active sites for H⁺ uptake, and a four-step reversible structural evolution occurs from PTO-NH₃⁺ to PTO-NH₃⁺-H, PTO-NH₃⁺-2H, PTO-NH₃⁺-3H, and PTO-NH₃⁺-4H, respectively. In addition, the introduction of –NH₃⁺ narrows the energy gap, which facilitates electronic conductivity and charge affinity. Owing to the fast kinetics of H⁺ coordination/de-coordination behavior, the PTO-NH₃⁺ electrode not only achieves an outstanding proton-storage capacity of 214.3 mA h g⁻¹ at 0.05 A g⁻¹, but also represents excellent rate performance of 112.9 mA h g⁻¹ at 40 A g⁻¹ and long-term cycle durability (capacity retention of 74.1% over 10 000 cycles at 10 A g⁻¹) in the 9.5 m H₃PO₄ electrolyte. This work provides a novel strategy for the engineering of organic small molecules for ultrafast proton storage and lays a solid foundation for further advancement of aqueous proton electrochemistry.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Zhejiang Province (LY23B030005), National Natural Science Foundation of China (22209082), and Natural Science Foundation of Ningbo Municipality (2023J098).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00269a

‡ These authors contributed equally to this work.

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